The present invention relates to a new and improved construction of device for facilitating the alignment or selective positioning of heavy workpieces at a machine table or the like.
Heavy workpieces, for the purpose of machining such at a machine tool, for instance hobbing, milling or grinding machines, are usually prepared in that they are directly deposited upon a machine table and chucked or fixedly clamped thereat. In many instances the exact machining or processing of the workpiece does not require any more than that the relevant tools are exactly aligned in relation to the fixedly clamped workpiece. However, there are situations where it is important that heavy workpieces are exactly aligned in relation to a certain point and/or a predetermined axis, for instance the rotational axis of the machine table of a gear working machine which operates according to the generating method. This alignment work is usually difficult and time-consuming, since the frictional forces between the heavy workpiece and the machine table or the like, are not only large, but owing to the known change-over between the static friction and the sliding friction, the so-called stick-slip-effect, are irregular. In addition thereto it should be appreciated that certain heavy workpieces, for instance large toothed rims, are not sufficiently inherently stable in order to withstand the displacement forces needed for overcoming the static friction, without deforming to such a degree that there is impaired the accuracy of the subsequent machining or processing work. It is conceivable to avoid this difficulty by providing ball or spherical guides between the machine table or the like and the workpiece, the spherical guides each comprising a lower plate and an upper plate supported thereon by means of balls or spheres. However, such ball guides, on the one hand, are sensitive to impacts which arise during the deposit or placement of the heavy workpieces, and, on the other hand, are not capable of being sufficiently loaded, in order to withstand the appreciable clamping or tensioning forces which are needed in many instances in order to positively fixedly retain the workpiece.